In the prior art, many methods are known for imparting dimensional stability and water repellency to wood and building materials such as wood. Typically, materials are coated or impregnated with solutions of silicone, acrylic, urethane, ester, fatty and oily resins or monomers, followed by drying. Of these repellents, silicone repellents are widespread. In particular, silicone water repellents of the solvent dilution type become the main stream.
However, water repellents of the solvent dilution type generally have a more negative influence of the solvent on the environment than the water dilution type. Also from the standpoints of environmental protection and resource preservation, there is a strong desire to have water repellents which do not use solvents, especially aqueous water repellents of high performance.
While many aqueous water repellents were recently developed, JP-A 1-292089, JP-A 5-156164 and JP-A 5-221748 disclose long term stable emulsions having alkyltrialkoxysilanes emulsified in water. However, these emulsions have several drawbacks since they use alkoxysilanes characterized by very slow hydrolytic reaction. When the emulsion is applied to a material, the material is effectively impregnated therewith, but the silane volatilizes off from the material surface. As a result, the material surface loses water repellency, becomes vulnerable to water wetting, staining and popup by frosting and thus undesirably less durable, and looks milky white on outer appearance.
JP-A 8-199066 and JP-B 7-39494 disclose methods for preparing emulsion-base water repellents of trialkylsiloxysilicates capable of imparting high water repellency. These repellents are expensive because trialkylalkoxysilanes or trialkylsilanols are used as the starting material. The preparation methods are complex and uneconomical. When an alkoxysilane is polymerized in an aqueous emulsifier solution at a temperature below 15° C., a uniform emulsion is not obtainable. The resulting trialkylsiloxysilicate-base emulsion water repellent is unsatisfactory.
Aside from the emulsion type mentioned above, JP-A 61-162553, JP-A 4-249588 and JP-A 10-81752 disclose water repellents of homogeneous aqueous solution type.
However, the composition of JP-A 61-162553 lacks storage stability in that rapid polymerization reaction takes place upon dilution with water. The composition must be used within a day after dilution and is thus impractical. The rapid polymerization reaction leads to a molecular weight build-up, which retards impregnation of the material therewith, sometimes leaving wet marks on the material surface.
The composition of JP-A 4-249588 comprising a water-soluble amino group-containing coupling agent and an alkyltrialkoxysilane having a short carbon chain has good storage stability, but poor water repellency probably because only the lower alkyl group contributes to water repellency. Since the amino group-containing coupling agent component is included in excess of the alkylalkoxysilane component as demonstrated by a molar ratio of alkylalkoxysilane component/amino group-containing coupling agent in the range from 0.5/10 to 3/1, there are problems that wet color marks are left on the material surface and paper, fibrous items and wood are substantially yellowed.
JP-A 2000-95868 discloses a method for preparing a composition by first partially hydrolyzing an alkyltrialkoxysilane or alkyldialkoxysilane having a short carbon chain and an amino group-containing alkoxysilane, adding hydrolytic water and an acid to effect further hydrolysis, and finally adding a neutralizing agent. This method is complex. In the first step of effecting hydrolytic reaction on a mixture of the alkylalkoxysilane and the amino group-containing alkoxysilane, the amino group-containing alkoxysilane generally has a higher hydrolytic rate than the alkylalkoxysilane, which becomes a bar against co-hydrolysis, failing to effectively form a co-hydrolytic product. The composition finally obtained by this method is thus unsatisfactory. Treatment of neutral substrates with the composition undesirably imparts poor water repellency.
JP-A 7-150131 discloses the treatment of wood with a composition comprising a salt of an organic or inorganic acid with a basic nitrogen-containing organopolysiloxane, a water repellent substance and water. This composition, however, has the problems of insufficient water repellency and storage instability.
JP-A 55-133466 and JP-A 55-133467 disclose a composition obtained by hydrolyzing an alkylalkoxysilane, an amino group-containing alkoxysilane, an epoxy group-containing alkoxysilane and a metal-metalloid salt with water. The treatment of substrates with the composition minimizes yellowing. However, since amino groups are blocked by the reaction of amino groups with epoxy groups, the composition becomes so difficultly soluble in water that it cannot be used as an aqueous treating agent. The amino blocking also restrains the adsorption of the composition to substrates so that the composition cannot be used for the treatment of substrates.
To solve the above problems, we proposed in JP-A 9-77780 a composition comprising the co-hydrolyzate of an alkylalkoxysilane having 7 to 18 carbon atoms, an alkoxy group-containing siloxane and an amino group-containing alkoxysilane. Despite the use of long chain alkyl silane, the composition provides substrates with weak water repellency. When paper, fibrous items and wood are treated with the composition, somewhat noticeable yellowing occurs.
Proposed in JP-A 10-081752 is a binder composition which is stable in an alkaline region. Due to a substantial amount of amino group-containing alkoxysilane used therein, this composition had many problems including insufficient water repellency as an agent for treating non-alkaline substrates, wet color left on the treated material, and substantial yellowing.
Accordingly, all the water repellents described above are seldom regarded as performing satisfactorily for the treatment of wood substrates originating from lignocellulose materials.
On the other hand, housing members available at present include plywood members which are often used as bearing wall members, structural floor sheathing members, and roof sheathing members, and veneer laminates which are often used as two-by-four members and Japanese traditional wooden framework members.
It has heretofore been possible to produce plywood and veneer laminates from a useful wood raw material having excellent properties which is selected for a particular purpose or application from among wood raw materials having relatively good properties, for example, south sea timber. Due to the depletion of wood resources, it is not always possible under the currently prevailing circumstances to use only a wood raw material having excellent properties. Now that the regulation of insuring and promoting the quality of houses and buildings has been enforced, the quality demand to housing members is and will be increasing. It is forecasted that the future need is to produce plywood or veneer laminates which are less expensive, have good physical properties and impose a less load to the environment upon discarding.
These facts suggest that with the progress of depletion of wood resources, the preparation of wooden panels from a wood material having excellent properties as the raw material is not always possible. In particular, plywood and veneer laminate products from a typical forested tree, Radiate pine (Pinus Radiata D. DON) as the raw material have not been widespread because of problems including dimensional changes, warping and mildewing due to their high water and moisture absorptive properties.
One conventional approach used to solve these problems is to apply emulsions of acrylic water repellents or paraffinic water repellents. However, a blocking problem often occurs when these water repellents are applied to plies and dried and the plies are piled up. This problem precludes widespread use in practical applications.